This invention relates to picture element (pixel) encoding, and, more particularly, to apparatus and method for increasing the resolution of a display by reducing pixel quantization effects.
In a typical display such as may be used in combination with a real-time computer image generation (CIG) system, an image of an object is presented to a viewer as a series of frames. Although the rate at which a frame is updated may vary, it is customary to use a 30 Hz update rate which corresponds to the update rate used for the NTSC television standard.
For a raster-scanned display, raster lines having a predetermined number of pixels per line are generated in response to video information generated by the CIG system. Pixels are generally square or rectangular portions of the display and are arranged in a grid. The lines of the raster may be presented in either an interlaced or non-interlaced format. In the interlaced format, two fields, each updated at say a 60 Hz rate, are used for presenting the respective odd and even lines of the frame. That is, after the lines of the odd field have been displayed, the lines of the even field are displayed between appropriate lines of the odd field. In non-interlaced format, the lines of the frame are generally sequentially presented.
The ultimate goal of a CIG system is to determine an attribute for an image to be displayed such as the color or shade of gray for monochrome system of each pixel of a display device, so that :;hen each pixel is displayed with its assigned color value, the desired image is formed on the display device. Descriptors of image features are typically stored as polygons, or faces. A common sampling scheme which assigns the color of a polygon to any pixel whose center is covered by the polygon as determined by a view ray from the observer through the pixel center to the polygons in viewer space results in aliasing, which may be manifested by a flashing effect along edges within the image.
One way to minimize aliasing is to supersample the pixel at subpixel locations. Each pixel is divided into a plurality of subpixels and the color of a polygon intercepted by a view ray passing through the center of the subpixel is assigned to the subpixel. After colors have been assigned to all subpixels, the color of the overall pixel may be determined, such as by averaging the colors of all subpixels of the pixel. Details of CIG systems for image processing may be had by reference to U.S. Pat. Nos. 4,727,365--Bunker et al; 4,811,245--Bunker et al; and 4,825,391--Merz, all assigned to the instant assignee and incorporated herein in their entirety by reference thereto.
In prior CIG systems, a continuum of colors may be displayed along a line of a raster subject to a restriction that color may not change faster than the bandwidth of the display device. For example, if a raster line contains 1024 pixels as supplied by the CIG system and the information from the CIG system is limited to changing only once for any pixel, then a color change, say from black to white or from white to black, in the space of less than one pixel cannot be displayed. However, generally there is no restriction placed on the display device as to where along the raster line a color change can occur, only a restriction on the rate or frequency of change. When a raster line is divided into pixels such as for accommodating output from CIG systems, then for previous CIG systems the raster would typically only change color at the start of each pixel. This limitation of color change location produces a quantization effect which does not always permit all color information to be accurately displayed, thereby reducing the modulation transfer function (MTF) of the system. In other words, when the color value of a pixel is allowed to change only at the beginning of a time interval (known as "pixel time"), which corresponds to the physical leading edge of a pixel and remain fixed throughout pixel time, MTF is limited.
The modulation transfer function is an indication of the resolution of a display system, or an indication of the quality of the rendition of an image obtained from a predetermined input value used to generate the image. Resolution may be regarded as the ability of a system to detect and/or display spaced apart edges. The closer edges are that may still be accurately detected or displayed, the higher the resolution.
Unless otherwise stated, white and black will be used as examples of contrasting colors throughout, wherein a color resulting from a combination of white and black is represented as shade of gray. It is to be understood that this invention is applicable to any colors including different intensities of the same color.
A standard test for determining MTF involves moving, or sampling, a bar pattern of alternating contrasting colors, wherein a bar of the pattern is about 1 to about 1.25 pixels wide, across the pixel grid and displaying the result. If the contrasting colors are say black and white, then a display of any color different from black or white, such as a shade of gray, represents a loss of information and a corresponding MTF of less than 100 percent.
It would be desirable to increase the MTF and thereby the resolution of image displays. Further, it would be desirable to effect such an increase with minimal hardware changes to existing computer image generation systems.
Accordingly, it is an object of the present invention to provide apparatus and method for increasing the modulation transfer function and thereby the resolution of image displays.
Another object of the present invention is to increase the MTF of image displays used in combination with a computer image generation system with minimal hardware changes to the computer image generation system.